One of the methods of improving the smoothness of paper/paperboard is to pass the paper/paperboard through one or more heated calendering nips which are at a temperature higher than the temperature of the web. The surface of the paper/paperboard that is to be finished is pressed against the heated roll. The applied heat raises the surface temperature of the paper/paperboard to the glass transition temperature, which causes the fibers to soften and conform to the surface of the roll. The gradient in the temperature tends to lower the glass transition temperature preferentially on the external surfaces of the paper/paperboard, allowing the sheet to achieve a desired smoothness without significant reduction in caliper. However, improved smoothness does not necessarily translate into improved printing performance.
U.S. Pat. No. 4,606,264 to Agronin discloses a method and an apparatus for temperature gradient calendering a web for obtaining improved gloss, smoothness, strength and ink transfer capabilities. By "temperature gradient calendering" is meant that there is a significant difference between the temperatures to which the web surface and the web interior are subjected. A web of paper is passed through a nip formed by an steel roll and a compliant roll. The steel roll is heated to a temperature in the range of 350-400.degree. F. The compliant roll can be made of NOMEX or other resilient material. When the web is compressed by the nip, the hot outer web surface is deformed more than the interior, resulting in a smoother, glossier and stronger web.
The Agronin patent states that it was known to perform standard calendering at a maximum working temperature of usually no more than 200.degree. F., using an iron roll and a compliant roll. In temperature gradient calendering, one or both of the rolls forming a nip is heated to a temperature of at least 350.degree. F. This temperature is "critical but dependent upon the `flow temperature` of the particular fibers of the web". In order to achieve effective temperature gradient calendering, the nip temperature must be sufficient to cause the surface fibers of the web to deform.
U.S. Pat. No. 5,163,364 to Bubik et al. discloses a method for calendering a paper or cardboard web. Calendering takes place in a calendering zone which operates under pressure with application of heat and moisture. The desired smoothness of the paper web is achieved with a long dwell time in a long calendering zone. A web which is still wet is guided between parallel heatable surfaces which are arranged on opposing sides of the web in facing relationship and pressed against the web. The surfaces are designed to form and hold a given precise contour of the calendering zone over its full length. One surface is constructed as a casing of a heated roll; the second surface is constructed as an endless flexible belt which can be pressed toward the heated roll by means of a supporting element which is concave toward the belt and which is of complementary construction to the radius of the roller casing. The endless belt is made of metal.
U.S. Pat. No. 5,251,551 to Abe et al. discloses a calendering apparatus in which a nip is formed by a chilled roll, a backing roll and an endless elastic belt arranged over the backing roll, whereby a paper sheet to be treated passes between the chilled roll and the endless elastic belt. The Abe patent asserts that a paper sheet having a remarkably improved smoothness and gloss can be obtained using the endless belt, without the disadvantage of a heat buildup. In the case of a prior art soft nip calender having an elastic roll constructed by covering a metallic roll with a layer of synthetic resin having a high thermal resistance, such heat buildup can lead to peeling at the boundary between the metallic roll and the synthetic resin layer, or to heat stress hysteresis in the synthetic resin layer, thereby shortening the working life of the elastic roll.
U.S. Pat. No. 5,400,707 to Neider et al. discloses a hot soft nip calendering apparatus for finishing a continuous sheet of paper. The apparatus includes at least one heated calender roll and a finishing belt which is moved in proximity to the heated calender roll by a plurality of drive rollers and at least one pressure roller. A heated calender nip is defined between the heated calender roller and the pressure roller. When a paper web is passed through the nip, one surface of the paper web is contacted by the heated calender roller and the other surface of the paper web is contacted by the finishing belt. The finishing belt has a smooth surface for contacting the paper web so as to impart appropriate smoothness and gloss characteristics to that surface. The finishing belt preferably comprises a woven substrate formed from a strong flexible synthetic material, such as KEVLAR, polyetheretherketone, RYTON or polyester. The belt further includes a finishing surface formed from a flexible elastomeric material, such as rubber or urethane, finished to a high degree of smoothness (e.g., 50 microinch).
PCT International Publication No. WO 96/28609 to Eriksson et al. discloses a coated paperboard for formed articles which is calendered after coating with a heated calender having a soft extended nip. The term "extended nip" is considered to comprise nip widths of 3 to 10 cm. [Note: In the present application, the term "extended nip" is broadened to encompass nip widths of 1 to 25 cm.] The paperboard consists of a fiber matrix in one, two or more layers and a coating. The calender is constructed to have a relatively soft elastic moving belt supported by a glide body or roll.
The linerboard product which is pertinent to the present invention contains more than one layer of fibers with the top layer being composed of recycled or virgin unbleached/bleached pulp. The plies below the top layer can be more than one and can be virgin kraft or recycled kraft including old corrugated containers. In some cases, the top ply of the multilayer linerboard can be coated with a pigmented or non-pigmented formulation to improve appearance. This product is primarily used as a liner with high visual appeal in corrugated containers. High compressive strength and good print quality are the primary required attributes for this product.
In accordance with a known grade of linerboard having two plies and calendered using a hard nip, gloss mottle and print mottle are apparent in the final printed product. Testing has shown that the mottle can be traced to the formation of the base ply. The hardnip calendering on the machine aggravates the gloss mottle at high nip loads that are required for achieving good smoothness. Conventional soft nip calendering with a deformable nip was considered as an option for alleviating this problem. However, pilot trials showed that conventional soft nip calendering was not successful in reducing the gloss mottling significantly. Thus, there is a need for a method of calendering which will be successful in reducing gloss mottle in multilayer linerboard.